MESOSCALE DISCUSSION 0122
NWS STORM PREDICTION CENTER NORMAN OK
1154 PM CST WED MAR 04 2015
AREAS AFFECTED...PORTIONS OF MIDDLE/SWRN TN...NWRN AL...CNTRL/NRN
MS...SRN AR...NRN LA...NERN TX
CONCERNING...WINTER MIXED PRECIPITATION
VALID 050554Z - 051200Z
SUMMARY...WINTER MIXED PRECIPITATION WILL CONTINUE TO OVERSPREAD A
BROAD REGION FROM NERN TX ENEWD TOWARD THE ARKLAMISS DELTA REGION
AND NEWD INTO THE TN VALLEY AREA. FREEZING RAIN AND SLEET ARE
EXPECTED TO OCCUR.
DISCUSSION...THE INTERACTION OF A MIDLEVEL VORT MAX EMERGING OVER
THE SRN PLAINS WITH THE CYCLONIC CIRCULATION SURROUNDING A SHORTWAVE
TROUGH FARTHER N IS SUPPORTING THE EWD SPREAD OF LARGE-SCALE ASCENT
FROM PARTS OF CNTRL TX TO THE ARKLATEX REGION -- PER RECENT WATER
VAPOR LOOPS. PRECIPITATION...INCLUDING THAT ASSOCIATED WITH
LIGHTNING-PRODUCING CONVECTION...CONTINUES FORMING WITHIN THIS
REGIME...WITH AREAS OF EARLIER-DEVELOPED PRECIPITATION SPREADING EWD
TOWARD PARTS OF E-CNTRL TX. ALL OF THIS ACTIVITY IS EVOLVING WITHIN
AN ANAFRONTAL FLOW REGIME...WITH CONTINUED EWD SPREAD OF
PRECIPITATION ANTICIPATED DURING THE OVERNIGHT HOURS.
WHILE SFC TEMPERATURES ARE PRESENTLY ABOVE 0C IN MANY
LOCATIONS...AMPLE COLD ADVECTION...COUPLED WITH NOCTURNAL COOLING
AND AN INFLUX OF DRIER AIR FROM THE N WITH RESULTANT WET-BULB
COOLING...WILL ALL CONTRIBUTE TOWARD PROMINENT SFC TEMPERATURE
REDUCTIONS DURING THE OVERNIGHT HOURS. THE 0C ISOTHERM PRESENTLY
EXTENDS FROM MIDDLE TN TOWARD SRN AR TO NEAR WACO TX...AND IS
FORECAST TO CONTINUE ADVANCING ON A SWD/SEWD TRAJECTORY. GIVEN THE
WEAKLY SLOPING NATURE OF THE BAROCLINIC ZONE NWD/NWWD...RESIDUAL
WARM AIR ABOVE THE SFC SAMPLED BY THE 00Z SHV AND BNA RAOBS WILL
RESULT IN PARTIAL TO COMPLETE MELTING OF DESCENDING HYDROMETEORS.
SLEET AND FREEZING RAIN SHOULD BECOME INCREASINGLY DOMINANT ACROSS
THE AREA THROUGH THE NIGHT OWING TO REFREEZING WITHIN THE
SFC-BASED...SUB-FREEZING...POST-FRONTAL COLD LAYER. THIS WILL LIKELY
EXTEND SWD TO PORTIONS OF THE INTERSTATE 20 CORRIDOR...ESPECIALLY
AFTER 08Z. FREEZING RAIN RATES OF 0.05 INCH TO LOCALLY 0.10 INCH PER
HOUR WILL BE POSSIBLE...HIGHEST IN AREAS OF CONVECTIVELY ENHANCED
PRECIPITATION. SLEET ACCUMULATION RATES OF ONE-TENTH TO ONE-QUARTER
INCH PER HOUR WILL ALSO BE POSSIBLE. SLEET IS EXPECTED TO BE
RELATIVELY MORE COMMON ACROSS PARTS OF MIDDLE TN INTO SWRN
TN...WHERE COOLING OF THE ELEVATED WARM LAYER WILL HAVE A TENDENCY
TO REDUCE MELTING OF DESCENDING HYDROMETEORS.
COMPARISONS WITH RECENT SFC OBSERVATIONS REVEAL MODEST WARM BIASES
IN SFC TEMPERATURE FORECASTS FROM RECENT RUNS OF THE RAP AND NAM.
THIS IS PARTICULARLY THE CASE INTO THE ARKLAMISS DELTA REGION AND
VICINITY WHERE LOCAL-FLOW CHANNELING ACCENTUATES NEAR-SFC COLD
ADVECTION IN VALLEY-PARALLEL...NLY FLOW. THIS SUBSTANTIATES THE SWD
EXTENT OF THE DISCUSSION AREA. WHILE WARM TEMPERATURES ACROSS SOME
OF THESE AREAS DURING THE PRECEDING DAY -- ESPECIALLY IN PARTS OF
LA/MS -- MAY INHIBIT INITIAL ICE ACCUMULATIONS OWING TO A WARMER
GROUND...ICE ACCUMULATIONS WILL BE OF PARTICULAR CONCERN ON ELEVATED
SURFACES AND METAL OBJECTS.
..COHEN.. 03/05/2015
ATTN...WFO...OHX...BMX...HUN...MEG...JAN...LZK...SHV...
LAT...LON 32729078 32199285 32189454 32609469 33509241 34499070
35798820 36588633 35868573 34468768 32729078